Press forming machines are used for punching press, drawing molding, die forging, injection molding, and so on. A press forming machine generally has one mold as a fixed mold and the other mold as a movable mold. A vertical press forming machine has a lower support stand, a plurality of columns supported by the lower support stand, an upper support plate held by the columns, and a slide plate which can reciprocate between the lower support stand and the upper support plate along the columns and has a molding space between the slide plate and the lower support stand. In the molding space, a fixed mold is provided on the lower support stand, a movable mold is provided on the lower surface of the slide plate, and a work piece is molded between the fixed mold and the movable mold. The slide plate is generally formed into a plane and is vertically moved by a driving mechanism. It is desirable to carry out molding while the movable mold is kept on a desired position relative to the fixed mold, for example, while the movable mold is kept horizontally and moved. Thus, the slide plate is moved while being kept horizontally, and the columns are formed with rigidity and a large thickness to prevent the slide plate from being inclined during molding. However, in some cases, distortion occurs on the slide plate and so on and inclination occurs due to a clearance between sliding parts. Thus, it has been necessary to correct the mold to compensate for the distortion and inclination.
The driving mechanism is mounted on the upper support plate, a drive shaft extends from the mechanism, and the tip of the shaft is engaged to the slide plate. A servomotor or a hydraulic cylinder is used as a driving source. In the case of a motor, the rotation of the motor is transformed to a vertical motion by a crankshaft and a cam and the rotation of the shaft is transformed to a vertical motion by a ball screw.
In some shapes of the work piece to be subjected to press forming, an offset load may occur on a mold, and a fixed mold and a movable mold or a slide plate may not stay horizontal to each other. Regarding a plurality of driving sources provided for driving the slide plate, the following proposal has been made: the slide plate is kept in a horizontal position by controlling the driving sources so as to maintain the synchronization among the plurality of driving sources.
However, since a work piece formed by press forming has a complicated shape such as a three-dimensional shape, force applied to the slide plate during molding is changed in the progress of the molding. Besides, a position where force is applied is shifted during the molding.
For example, FIGS. 9(A), 9(B), and 9(C) schematically show the state of reaction force applied to the slide plate when an oil pan for an automobile is formed by press molding. In these drawings, a slide plate 40 is indicated as x-y coordinates. For example, when molding is started, a cope initially reaches a drain of the oil pan and the drain is formed. Hence, force occurring thereon is applied to a fourth quadrant of the x-y coordinates. An oil dish is formed as the molding proceeds. Thus, large forces w2 and w3 are received from a second quadrant and a third quadrant of the coordinates. Force w1, which is originally applied, is reduced at this moment, and large force w4 on a first quadrant is added. Hence, composite force W is applied to the third quadrant. As the molding proceeds, the forces w2 to w4 decrease, force w5 is added, and composite force is applied to the right of y axis substantially on x axis.
The above application of forces and composite force, the magnitude of force, and the above changes in force may vary depending upon the shapes of the work piece and a traveling speed of the mold. The position and magnitude of the composite force, which is applied on the slide plate, is generally changed as the press forming proceeds.
As described above, the position where composite force is applied moves not only along a straight line but also in a biaxial direction, that is, on a plane when a work piece having a three-dimensional shape is molded.
When composite vertical force exerted to the slide plate is applied to the center, rotation moment for inclining the slide plate is not applied to the slide plate. Since the position where force is applied is moved as described above, the position and magnitude of rotation moment applied to the slide plate are also changed. Therefore, deformation occurring on the press forming machine is changed as the press forming proceeds. The deformation includes the extension and bending of the columns of the press forming machine and distortion of the slide plate, the upper support plate, and the fixed support plate during press forming.
In this manner, the application of load is changed as the press forming proceeds, and the extension and deformation on the parts of the press forming machine are also changed.
Conventionally, in order to minimize the extension and deformation on the parts of the press forming machine, that is, for example, in order to reduce the inclination and distortion of the slide plate, the slide plate increases in thickness with rigidity and the columns increase in thickness to reduce a gap between the slide plate and the columns. And then, when a plurality of driving sources is used to press the slide plate, a main driving source is driven according to a desired control style to move down the slide plate, and the other slave driving sources are driven while being controlled according to the descend of the main driving source.
The controlling method using the main driving source and the slave driving sources is a method for evenly pressing the entire of the slide plate (e.g., while being forcefully kept in a horizontal position) while the rigidity of the slide plate is made sufficiently large. This method is effective for a large press forming machine.
However, when distortion on the parts of the slide plate and other parts of the machine needs to be considered, in the method for performing driving while controlling the slave driving sources according to the main driving source, in view of the above-mentioned distortion, it is extremely difficult to allow the slave driving sources to follow the main driving source such that the distortion is eliminated. Further, even when the above-mentioned method is possible, in view of control exercised by a computer when the main driving source and the slave driving sources are controlled, a processing amount of the computer is extremely large, so that it is necessary to install a high-speed computer.
An object of the present invention is to provide a press forming machine which can separately drive driving sources so as to always maintain a movable mold at a desired position relative to a fixed mold when press forming proceeds.
Another object of the present invention is to provide a press forming machine whereby when the same kind of work piece is repeatedly subjected to press forming, control data corresponding to driving sources is previously stored in a memory of control means in each of a plurality of operating steps, and the driving sources are driven separately in an asynchronous manner according to the stored control data during press forming so as to perform desired molding.
As a result, molding time can be shortened in the case of repeated molding. Even when a CPU of the control means is relatively slow in processing speed, the driving sources can be controlled, thereby reducing molding time.